1. Field of the Invention
The present invention relates to a body coated with a diamond (hereinafter referred to as a diamond-coated body ) and a process for the preparation thereof and, more particularly, to a diamond-coated body and a process for the preparation thereof, the diamond-coated body having excellent adhesion of a film of the diamond (hereinafter referred to as a diamond film) to a substrate, demonstrating high performance and excellent durability when applied.
2. Description of Related Art
Heretofore, super hard alloys, sintered diamond, single crystal diamond and so on have been employed for tools requiring a high degree of hardness and high abrasion resistance, such as cutting tools and dies.
Among those, diamond tools are particularly preferred due to their excellent properties such as hardness and abrasion resistance.
As diamond tools, there have been employed ones prepared, for instance, by brazing a sintered diamond or a single crystal diamond to the surface of such a substrate as composed of, for example, a super hard alloy or a highly hard metal.
Recently, review has been made on processes for preparing diamond-coated bodies by allowing a diamond film to be deposited on and coating the surface of the substrate composed of the super hard alloy or highly hard metal through the vapor phase diamond synthesis technology such as CVD or PVD methods, and attempts have been made to apply the resulting diamond-coated bodies to those uses as described hereinabove.
It is to be noted, however, that diamond is the hardest substance so that the diamond film to be deposited the surface of the substrate consisting of the super hard alloys or the like is considered to be effectively employed as a coating material or a protective film for providing the substrate with a high degree of hardness and abrasion resistance.
It is thus considered that super hard tools with further improved performance can be prepared, for example, by coating the surface of the substrate consisting of the super hard alloy or the like to be employed for the super hard tools such as cutting tools, dies and so on.
It is to be noted, however, that the adhesion between the surface of the super hard alloy and the diamond film is generally poor and no tools that can withstand actual application have been prepared yet.
Some technology has been proposed in which an intermediate layer is formed between the surface of the super hard alloy and the diamond film with the attempt to improve adhesion between them.
For example, Japanese Patent Laid-open Publication (kokai) No. 126,972/1983 discloses a super hard alloy with a diamond film, which can be prepared by first coating the surface of the super hard alloy with an intermediate layer selected from at least one material selected from the group consisting of a carbide, nitride, boride or oxide of a metal belonging to group IVa, Va or VIa and then coating the surface of the resulting intermediate layer with a diamond film.
As is apparent from the description made immediately hereinabove, the process disclosed in the aforesaid patent laid-open publication adopts a two-step process comprising the first step of coating the surface of the super hard alloy with the intermediate film and the second step of coating the intermediate film with the diamond film, so that this process is said to be laborious in the process for preparation. Further, this process cannot be said to achieve improvement in adhesion of the diamond film to the super hard alloy to a sufficient extent and to a practically applicable level, although the aforesaid patent laid-open publication claims so.
Further, there has been proposed technology for improving adhesion between the substrate consisting of the super hard alloy or the like and the diamond film without forming any intermediate layer.
For instance, Japanese Patent Laid-open Publication (kokai) No. 100,182/1988 discloses a super hard alloy with a diamond film prepared by coating a tungsten carbide type super hard alloy consisting of tungsten carbide in a particular particle size range and containing a particular amount of Co with the diamond film.
The resulting super hard alloy with the diamond film as disclosed in this patent publication, however, cannot be said to demonstrate a sufficiently practical level of adhesion between the super hard alloy and the diamond film.
In particular, if the amount of Co to be added would be increased, the thermal expansion coefficient of the substrate becomes greater and further carbon may be dispersed into Co, thereby making the favorable coating with the diamond film difficult. As a result, the adhesion between the substrate and the diamond film is decreased thereby failing to achieve an adequate degree of durability.
It is generally said that, if the thermal expansion coefficient of the substrate would differ from that of the diamond film to a considerably great extent, it is considered that a great degree of thermal stress may occur within the diamond film upon cooling after coating and this thermal stress works as the cause of a decrease in the adhesion, thereby making it likely to cause damages such as separation of the diamond film from the substrate when employed as a super hard tool.
Recently, in order to improve the adhesion of the diamond film to the substrate with the above matters taken into consideration, extensive development and selection of such a substrate of a new type has been made as consisting of hard materials, particularly such as ceramics (sintered bodies ), having the thermal expansion coefficient close to that of diamond and having the likelihood of being directly coated with the diamond film.
For instance, proposals have been made on attempts to provide diamond-coated bodies having the diamond film coated with high adhesion by using, as the substrate, Si.sub.3 N.sub.4, ceramics (sintered bodies) containing Si.sub.3 N.sub.4 as a major component or super hard substrate having controlled thermal expansion coefficient, many of such super hard substrates being a Si.sub.3 N.sub.4 sintered body or ceramics based on silicon nitride. It can be noted herein that it is known various properties, such as the thermal expansion coefficient, of silicon nitrides may vary with sintering conditions and addition of TiN, TiC, ZrN, SiC, ZrO.sub.2, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 or the like (as disclosed, for example, in Japanese Patent Publication (kokoku) No. 59,086/1985 and Japanese Patent Laid-open Publication (kokai) Nos. 122,785/1985, 109,628/1986, 252,004/1986, 291,493/-1986, 107,067/1987, 20,478/1988, 20,479/1988, 33,570/1988, and 306,805/1988).
It is to be noted that the diamond-coated bodies prepared by using those conventional substrates as disclosed in those prior patent publications still have the problems that the adhesion between those conventional substrates and the diamond film is inadequate and they do not have performance, particularly durability, to such a sufficient extent as being required by super hard tools. The problems specifically include a short cutting life when they are employed as cutting tools.